Fluor Turquoise vs Fluorescent Blue

How close are they?

ΔE 54.6, loose: different enough that you should pick on purpose. Fluorescent Blue is more saturated, cooler than Fluor Turquoise.

Can I use Fluorescent Blue instead of Fluor Turquoise?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 55). Fluorescent Blue is more saturated, cooler than Fluor Turquoise.

Can I use Fluor Turquoise instead of Fluorescent Blue?

The same difference applies the other way round: Fluor Turquoise is more muted, warmer.

ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.